Di-phase pickling process



July 16, 1963 A. 1.. F. HYDEN 3,

DI-PHASE PICKLING PROCESS Filed Sept. 26. 1960 ATTORNEYS United StatesPatent 3,097,972 DI-PHASE PICKLING PROCESS Anders Lennart Fredrik Hydn,Farsta, Sweden, assignor to Aktiebolaget Atomenergi, Stockholm, Sweden,a company of Sweden Filed Sept. 26, 1960, Ser. No. 58,223 Claimspriority, application Sweden Oct. 19, 1959 2 Claims. (Cl. 134-28) Thisinvention relates to pickling processes in general, i.e. processes oftreating a metal object with a liquid, usually an acid, for affectingthe surface of the metal, usually for removing metal oxides. Thefollowing specification refers to one particular pickling process, butit is evident to any body skilled in the art that the invention is notlimited to that particular process.

In many pickling processes it is important that the objects to bepickled are transferred as rapidly as possible from the pickling bath tothe subsequent rinsing bath. One example of such a process is thepickling of objects of zirconium and zirconium alloys in a mixture ofnitric acid and hydrofluoric acid, for the purpose of removing from thesurfaces of the objects such impurities as might othenwise producedamage due to corrosion. If such objects are lifted from the picklingbath in the conventional way and are then lowered into the rinsing bath,this must be done within a few seconds, because otherwise there areprecipitated on the metal surface chemical com.- pounds which arediflicult to dissolve and to remove by rinsing, and which jeopardize theresult of the pickling.

The present invention relates to a method which makes it possible totransfer the objects from the pickling bath to the rinsing bath directlyand without transporting them in the air. The method of the invention ischaracterized by arranging a pickling liquid, having one specificweight, and a rinsing liquid, having a different specific weight, asstratified liquids, the lighter liquid (forming a layer upon the heavierliquid, treating the object to be pickled in the pickling liquid andthen transferring it into the rinsing liquid through the boundary layerbetween the two liquids.

The rinsing liquid is usually water, and water is lighter than thepickling liquid and is consequently stratified to form a layer upon thepickling liquid. It is possible, however, to stratify a lighter picklingliquid to form a layer upon a heavier rinsing liquid, for instance acompartively concentrated sulphuric acid solution. In this case theobject must not, of course, after having been rinsed, be lifted upthorugh the overlaying pickling liquid layer, but must be removed fromthe apparatus while entirely surrounded by the rinsing liquid, forinstance through a U-shaped chamber.

The pickling of objects of zirconium and its alloys may be carried outin a vertical cylinder. The lower portion of the cylinder is filled withan aqueous solution containing 39% by volume nitric acid having aspecific gravity of 1.42, and 3.5% by volume of a hydrofluoric acidsolution containing 52% by weight hydrofluoric acid. Water is stratifiedto form a layer upon the acid solution. The objects to be pickled arelowered through the open top end of the cylinder, through the waterlayer into the pickling liquid. When the pickling is finished theobjccts are lifted into the water layer where they are rinsed, forinstance for a time of five minutes. The rinsing is 3,097,972 PatentedJuly 16, 1963 preferably made in running water. This can be arranged byadding water to the upper portion of the water column, and withdrawingwater from the lower portion of said col urnn, near the boundary layerbetween the pickling liquid and the rinsing liquid. If desired thepickling liquid, too, can be changed continually, by adding picklingliquid to one end of the pickling liquid column, and withdrawingpickling liquid from the other end of the column.

The above described process and the apparatus referred to areillustrated in the accompanying drawing which is a front view of thevertically disposed cylindrical vessel. Referring to the drawing, 1 isthe rinsing zone, 3 is the pickling zone and 2 is the dividing zone inwhich the boundary layer between the pickling and rinsing liquids ismaintained. Water is passed through the rinsing zone 1 by introducingfresh water at pipe 4 and withdrawing water contaminated with picklingliquid at pipe -5 which is provided with an open tube 6, preferablytransparent, for observing the level of liquid in the zone 1. Theoverflow pipe 7 serves to prevent the liquid from overflowing the top ofthe vessel. The pickling zone 3- is doublewalled or jacketed and iscooled by introducing water into the jacket through the pipe 10 andwithdrawing it through the pipe 1. Pickling liquid is introduced throughthe pipe 9 and the three thermometers 8, 8, 8 serve for observation andcontrol of the temperature of the pickling liquid. The apparatus may beused for either batch or continuous operation. In batch operation zone 3is filled with acid up to zone 2 while a flow of water is maintainedthrough zone 1. When the pickling acid becomes exhausted it is withdrawnthrough pipe 9 and zone 3 is then refilled with fresh pickling acid. Incontinuous operation acid is introduced continuously or intermittentlythrough the pipe 9 and is permitted to rise to the level of the outletof pipe 5 and to overflow therethrough along with the rinsing water fromzone 1. Thus the boundary layer or mixed zone is maintined in the zone2. No difficulty is encountered in keeping the rinsing layer and thepickling layer separate. In the continuous operation described above theupper boundary of the mixed zone will of course rise to the level of theoutlet to pipe 5.

The pickling and rinsing liquids have a tendency of getting mixed witheach other at the boundary layer between the two liquids, viz. bydiffusion and by the movements caused by the objects passing through theboundary layer. It has been found, however, that it is possible tomaintain the boundary between the two liquids sutficiently sharp forpractical purposes. It is possible to control the height of the mixedzone, for instance by continually changing the pickling and rinsingliquids, as suggested above. In this case, the height of the mixed zoneis determined by the distance between the two levels at which thepickling and rinsing liquids are withdrawn from the apparatus. Theposition of the mixed zone may be checked by means of bodies which floatin the liquid and which have known specic weights between that of thepickling liquid and that of the rinsing liquid. These bodies placethemselves at a level where the specific weight of the liquid is equalto that of the body.

What is claimed is:

1. A method of pickling objects of zirconium and zirconium alloys whichcomprises establishing a stratified bath consisting of a heavier bottomlayer of an aqueous solution of nitric acid and hydrofluoric acid, alighter top layer of Water and a boundary layer of a mixture of theliquids of said top and bottom layers, the liquids of said layers beingmiscible with each other in all proportions, supplying Water to said toplayer adjacent to the upper surface thereof and Withdrawing Water mixedwith said aqueous solution from said top layer at a point adjacent tosaid boundary layer, and moving said objects from said bottom layerthrough said boundary layer into said top layer.

2. A method as defined in claim 1 in which said bottom layer ismaintained by adding said solution thereto near the lower surfacethereof.

4 References Cited in the file of this patent UNITED STATES PATENTSMiskella Feb. 3,

BOOth Feb. 9,

Campbell Apr. 30,

Engelhardt July 14,

KBurg Feb. 2, 1960 FOREIGN, PATENTS Canada June 9,

1. A METHOD OF PICKLING OBJECTS OF ZIRCONIUM AND ZIRCONIUM ALLOYS WHICHCOMPRISES ESTABLISHING A STRATIFIED BATH CONSISTING OF A HEAVIER BOTTOMLAYER OF AN AQUEOUS SOLUTION OF NITRIC ACID AND HYDROFLUORIC ACID, ALIGHTER TOP LAYER OF WATER AND A BOUNDARY LAYER OF A MIXTURE OF THELIQUIDS OF SAID TOP AND BOTTOM LAYERS, THE LIQUIDS OF SAID LAYERS BEINGMISCCRIBLE WITH EACH OTHER IN ALL PROPORTIONS, SUPPLYING WATER TO SAIDTOP LAYER ADJACENT TO THE UPPER SURFACE THEREOF AND WITHDRAWING WATERMIXED WITH SAID AQUEOUS SOLUTION FROM SAID TOP LAYER AT A POINT ADJACENTTO SAID BOUNDARY LAYER, AND MOVING SAID OBJECTS FROM SAID BOTTOM LAYERTHROUGH SAID BOUNDARY LAYER INTO SAID TOP LAYER.